Electrically erasable and programmable read-only memories (EEPROM) are used wherever reprogrammable nonvolatile memory is required. Typically, writing to such devices requires asserting a write enable signal to the chip at the same time the write operation takes place. This prevents inadvertent writing of the memory.
Data contained in an EEPROM, however, is susceptible to various sources of corruption. For example, transients due to powering up and powering down an EEPROM present an opportunity for data corruption. EEPROMs typically find application in harsh industrial environments, thus exposing the devices to noise spikes on the control lines. It is therefore desirable to provide enhanced protection against inadvertent writes, in addition to the simple write enable signaling presently used.
EEPROMs also find use in situations where controlled read access is desirable. For example, smart cards incorporate EEPROM-type memory which require some form of protection against unauthorized access. Such cards are used in personal banking applications, health delivery services, and so on where privacy of the information contained in the card is fundamental.
EEPROMs can be found in radio frequency identification devices (RFIDs), where the memory device is used to store information identifying the object to which an RFID tag is attached. Typically, RFID tags can be written in order to store information in addition to an identifier. RFID tags usually have some sort of write protection capability and read access control. Such read access control is currently implemented by providing password mechanisms which gate access to the memory incorporated in the tag, resulting in a bulky device.
What is needed is a strategy which prohibits reading some or all of the information contained in a memory device, as well as write access to the memory. It is desirable to avoid having additional circuitry to implement such capability, thus permitting smaller and more compact applications which require read access protection.